linux-kernel - Re: [PATCH v2] rpmsg: virtio: don't let virtio core to validate used length

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Date:   Wed, 24 Nov 2021 16:12:26 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Arnaud Pouliquen <arnaud.pouliquen@...s.st.com>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Ohad Ben-Cohen <ohad@...ery.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        linux-remoteproc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH v2] rpmsg: virtio: don't let virtio core to validate used
 length

On Wed, Nov 24, 2021 at 05:20:45PM +0100, Arnaud Pouliquen wrote:
> Using OpenAMP library on remote side, when the rpmsg framework tries to
> reuse the buffer the following error message is displayed in
> the virtqueue_get_buf_ctx_split function:
> "virtio_rpmsg_bus virtio0: output:used len 28 is larger than in buflen 0"
> 
> As described in virtio specification:
> "many drivers ignored the len value, as a result, many devices set len
> incorrectly. Thus, when using the legacy interface, it is generally
> a good idea to ignore the len value in used ring entries if possible."
> 
> To stay in compliance with the legacy libraries, this patch prevents the
> virtio core from validating used length.
> 
> Fixes: 939779f5152d ("virtio_ring: validate used buffer length")
> 
> Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@...s.st.com>
> Cc: Jason Wang <jasowang@...hat.com>
> Cc: Michael S. Tsirkin <mst@...hat.com>
> ---

Arnaud, thanks a lot for the analysis.

Jason, I think this is another good point. We really should not
validate input for legacy devices at all.


> Update vs v1[1]: update commit message to clarify the context.
> 
> base-commit: fa55b7dcdc43c1aa1ba12bca9d2dd4318c2a0dbf
> 
> [1]https://lore.kernel.org/lkml/20211122160812.25125-1-arnaud.pouliquen@foss.st.com/T/
> ---
>  drivers/rpmsg/virtio_rpmsg_bus.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/rpmsg/virtio_rpmsg_bus.c b/drivers/rpmsg/virtio_rpmsg_bus.c
> index 9c112aa65040..5f73f19c2c38 100644
> --- a/drivers/rpmsg/virtio_rpmsg_bus.c
> +++ b/drivers/rpmsg/virtio_rpmsg_bus.c
> @@ -1054,6 +1054,7 @@ static struct virtio_driver virtio_ipc_driver = {
>  	.feature_table_size = ARRAY_SIZE(features),
>  	.driver.name	= KBUILD_MODNAME,
>  	.driver.owner	= THIS_MODULE,
> +	.suppress_used_validation = true,
>  	.id_table	= id_table,
>  	.probe		= rpmsg_probe,
>  	.remove		= rpmsg_remove,
> -- 
> 2.17.1